The present invention relates to apparatus for controlling the tension in a traveling textile yarn or like advancing strand material. As used herein, the terms "yarn," "strand" and "strand material" are to be interpreted in their broadest sense to generically encompass these and all other similar continuous length materials, including, but not limited to, thread, string, filament, fiber, cord, rope, wire, tape, ribbon, sliver, or other yarn-like or strand-like material.
In virtually all systems involving the handling of strand materials, it is a characteristic requirement that the tension conditions in the material be controlled in order to best insure high quality results. This is particularly true in the handling of traveling yarns in typical textile manufacturing systems. Conventionally, the control of strand tension in such operations has been commercially achieved by imposing an essentially fixed restraint, drag or load on the advancing strand. Various mechanical devices are well known for exerting a frictional drag on an advancing strand to perform this manner of tension control, including, for example, opposed disc pairs biased toward one another between which the strand passes, and interdigitating fingers or successively arranged pulleys or rollers for causing an advancing strand to travel in a sinuous path. In recent years, in order to avoid the perceived disadvantages of frictionally contacting a traveling strand, various alternative devices have been proposed wherein strand material is trained about a wheel or capstan to which a controlled braking force is applied magnetically, electromagnetically or by similar means to tension the strand material.
While these various devices differ significantly in their structures and manners of operation, virtually all operate on the basic principal of controlling strand tension by the imposition of an essentially fixed restraining force on the strand material and therein all such devices suffer the common fundamental inability to accommodate and offset fluctuations in the tension conditions in the strand material which naturally occur. For instance, in textile yarn handling systems, it is widely known to be a natural characteristic of yarn wound in packages to vary greatly in winding tension over the entire length of the yarn and that varying other forces may be imposed on the yarn during handling, which in combination create widely and sometimes radically fluctuating tension conditions in the yarn. Given these varying yarn tension conditions, tension controlling devices such as those described above provide only moderate effectiveness in maintaining yarn tension within a desirable range and are essentially effective primarily in merely maintaining a minimum tension in the yarn. Specifically, the restraint imposed by these devices on the traveling yarn is effective to compensate for downward tension fluctuations by maintaining a minimum restraint against yarn travel. However, such devices effectively magnify upward tension fluctuations in the yarn, rather than compensating for and offsetting such fluctuations, sometimes resulting in yarn breakage. Furthermore, while in many operations, it would be optimally preferred that as little tension as possible be maintained in a traveling yarn or other strand material, these devices typically impose a relatively significant tension in the stand material.
The magnetically and electromagnetically operated devices are reputed to be capable of more accurate setting to desired tension levels than mechanical tension devices so that the restraint imposed by these devices theoretically can be better regulated to minimize the deleterious effects of imposing a fixed restraint on a traveling strand. One such device disclosed in White U.S. Pat. No. 3,797,775, issued Mar. 19, 1974, entitled "Strand Tension Control," proposes an embodiment thereof in a closed loop control arrangement wherein the electromagnetically imposed restraint is continuously modified in response to tension variations detected by an electrically-operated strand sensing device to increase or decrease the restraint imposed to attain a desired constant tension condition. However, in actual practice, this device has not proved to be reliably operable in a commercial setting. Furthermore, such devices are significantly more complex and expensive than mechanical tension control devices and are not widely considered to perform any more satisfactorily in actual commercial use than mechanical devices. Accordingly, mechanical devices such as those described above, being simpler and less costly, are most widely used in industry despite their inability to compensate for and offset fluctuations in strand tension.
Accordingly, it is an object of the present invention to provide a strand tension control device of simple and inexpensive construction which is mechanically operable to monitor tension fluctuations in a traveling strand and to impose a varying frictional tension drag thereon in direct relation and response to tension fluctuations.